"The issue is not whether I can raise as much money or go ahead in the polls," the six-term senator said in a conference call with reporters. "It's my ideas. That's what this campaign is going to get to."

But Biden, whose 1988 campaign ended with charges of plagiarism, immediately found himself at the center of a new controversy, having to explain comments he made this week in which he criticized his rivals in the Democratic presidential field.

In an interview with the New York Observer, published Wednesday, Biden sharply attacked Clinton and former Sen. John Edwards of North Carolina for their plans for Iraq.

He drew even more scrutiny for calling Obama "the first mainstream African American who is articulate and bright and clean and a nice-looking guy."

By using the word "clean," Biden later explained, he meant only that Obama was "fresh" and "new." But the comment drew calls for an explanation.

"I do not think he meant to say anything intentional that was off-color, but it certainly is highly suggestive," the Rev. Jesse Jackson, himself a former Democratic presidential candidate, said on CNN.

Obama put out a statement challenging another part of Biden's comments.

"African American presidential candidates like Jesse Jackson, Shirley Chisholm, Carol Moseley Braun and Al Sharpton gave a voice to many important issues through their campaigns, and no one would call them inarticulate."

By the end of the day Biden had called Obama "a superstar, the most exciting candidate this party has had in a long time." In a written statement, Biden said: "I deeply regret any offense my remark in the New York Observer might have caused anyone. That was not my intent and I expressed that to Sen. Obama."

Biden, 64, first won election to the Senate in 1972, at 29. A month after his election, his wife and young daughter were killed in a car crash when a tractor-trailer broadsided their car.

Biden, who was in Washington at the time, considered withdrawing from the Senate to take care of his two sons, who were injured in the crash. To this day, he does not drive the 80 miles from Delaware to Washington but commutes by train. He has since remarried.

In the Senate, Biden has built a reputation as a foreign policy specialist. He is chairman of the Foreign Relations Committee and is the author of a Senate resolution opposing President Bush's escalation of troops in Iraq.

Biden ran for the 1988 presidential nomination but withdrew amid accusations that he had plagiarized portions of speeches by a British politician. Asked Wednesday what he learned from the experience, Biden said, "I took a real gut punch. I learned how to take it, and I got back up."

Last year, he was forced to defend a remark that "you cannot go to a 7-Eleven or a Dunkin' Donuts unless you have a slight Indian accent." He said his words had not been fairly portrayed.

In the New York Observer interview, Biden challenged the foreign policy prescriptions of other Democratic candidates in the 2008 race.

Asked about Edwards' call to immediately withdraw a portion of the U.S. force from Iraq, Biden said: "I don't think John Edwards knows what the heck he is talking about."

He said Clinton's plan to cap the number of U.S. troops and to threaten to cut funds to Iraqi leaders would be "nothing but disaster."

"This administration's mishandling of the war in Iraq may be the greatest foreign policy disaster of our time," Biden said in a video statement on his campaign website Wednesday. "Above all else, that's why I'm running for president."